The present invention relates generally to spring return cylinder actuators and more specifically to reversible spring return cylinder actuators.
A spring return cylinder actuator generally includes a piston in a cylinder which extends or retracts a piston rod connected to the piston based on the pressure at the ports. The return spring is used to return the piston to a no pressure position. The spring return cylinder actuator, through its piston rod, controls the position of an operable device. When connected to a valve, the cylinder actuator controls the position of the valve either between open and closed, or can control it to varying positions therebetween. Upon failure of the fluid pressure system, be it hydraulic or pneumatic, to the cylinder actuator, the return spring forces the piston rod, and consequently the device to be controlled, to a fixed, no pressure position. Using a valve for example, the desired failure, or no cylinder actuator pressure position of the valve will either be opened or closed depending upon the system in which the valve is installed. Thus, the spring return of the cylinder actuator must be designed specifically for the system so that the device to be controlled by the actuator fails in either a piston rod extended or piston rod retracted position.
In the prior art, the user would have to stock two types of spring return cylinder actuators; one for the failure in the extended piston rod position, and one for the failure in the retracted piston rod position. In an attempt to reduce the amount of inventory, reversible spring return cylinder actuators have been designed. In order to reverse the operation in these devices, the cylinder itself had to be disassembled with the piston, piston rod and spring being removed and placed in a different order with respect to each other and then reinserted into the cylinder housing to convert between a no pressure, extended and a no pressure, retracted embodiment. Not only is this assembly and reassembly time-consuming, but it also offers the opportunity to damage the seals on the cylinder and the piston. Also, depending upon the environment, it may be very difficult to disassemble the cylinder because of corrosion.
If a powerful return spring is used, an uneven spring force on the piston may result, causing it to cock or tilt. This cocking or tilting will decrease the life of the piston seals and provide hysteresis in the position of the actuator in response to the fluid signals.
Thus, it is an object of the present invention to provide a spring return cylinder actuator which may be converted between extended and retracted, no pressure positions without disassembly of the cylinder/piston assembly.
Another object of the present invention is to provide a spring return cylinder actuator with longer life.
A still even further object of the present invention is to provide a spring return cylinder actuator with minimum hysteresis.
An even further object of the present invention is to provide a spring return cylinder actuator with the strongest possible spring and the minimum side loading of the piston.
These and other objects are achieved by providing a pair of piston rods connected to the piston and extending exterior the housing with a spring biasing the piston to a no pressure position. By providing attachment to the exterior end of each rod, as well as mountings on each end of the cylinder housing, the device may be mounted with either piston rod and at either end of the housing such that depending upon which piston rod is attached to the device to be actuated, either an extended or retracted no pressure spring return cylinder actuator will be defined.
The two end walls of the housing each include a bushing through which the respective piston rods extend. This stabilizes the piston and prevents canting under the heavy spring loads, thereby extending the life and reducing the hysteresis. One of the bushings include a stop defining the no pressure position. This stop is threadably received in its end wall and adjusts the preloading or precompression of the spring. A rod cover is mounted to the end of the housing which is not to be mounted to the device to be actuated at the mountings to cover the piston rod which is not to be connected to the device to be actuated. The mountings are identical on both sides of the housing and include identical patterns of threaded bores to receive fasteners to mount the housing to the device to be actuated and to mount the Piston rod cover to the housing. The piston rods are sized such that one of the piston rods extends substantially further beyond the housing than the other when the piston is in the no pressure position.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.